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Eliminating RFI/EMI Interference from your Audio System

by Dan Banquer September 02, 2004
Radio Tower

Radio Tower

I PRESENTLY HAVE my listening space in the attic of my house and less than 100 yards from a rather large and reasonably powerful ham radio antenna that points in my direction. I don't enjoy listening to my neighbor's ham radio transmissions through my stereo system, so I decided to take whatever measures necessary to get rid of the problem.

Attacking and controlling this situation, however, was not a one-step operation. In fact, I had to take several steps to control both the radio frequency and electromagnetic interference (RFI and EMI) that was conducted through the a.c. lines in my house and radiated through the air.

Rx for Line Noise

I'll start with conducted interference first. The most important part of getting rid of conducted interference is to establish that you have a good, solid earth ground. Earth ground is usually tied to a large water pipe in the basement of your house or apartment building. Having a local electrician check this should be the first item on your list.

If you happen to own a voltmeter, checking the a.c. lines with your meter could well be the next step. To do this, simply connect the "ground" lead of the meter to the earth "ground" pin of your a.c. socket, then connect the other meter lead to the "hot" pin of your a.c. line. You should read between 115 to 125 volts a.c.

Next, move your "hot" meter lead to the "neutral" pin of the ac. line. You should read between 0 and 5 volts a.c., depending on how much your line is being loaded. If you aren't getting any readings at all, recheck your connections, and if you don't have a three-prong a.c. socket, you may not get any readings at all. Establishing that you have an earth "ground" connection available at your wall socket is critical, because all reputable a.c. line filters require the earth "ground" connection in order to work at their maximum effectiveness. It you don't have a three-prong AC wall socket, GET ONE INSTALLED!

The best overall surge-suppresser and RFI -EMI filter I have used so far is the Tripplite Isobar unit. Eight outlets of heavy-duty protection can be had for less than $75.00 from Computer Discount Warehouse and other such places. Will this affect the sound of your stereo? You bet! Most people have said that the noise floor dropped like a stone, and the "sound" of a large voltage spike on the a.c. lines is definitely not conducive to good sound and can be very damaging to your wallet for the repair bills from damaged equipment.

Radiated interference can prove to be a much tougher problem to cope with, but with a little work you can overcome this difficulty too. I have found that using three conductor microphone cable for my analog interconnects has been very effective. They are typically constructed with two leads in a twisted pair with an additional ground shield covering the twisted pair. To use this type of cable for "unbalanced" interconnects, simply do the following:

Connect one of the leads of the twisted pair to the ground shield at both ends of the of the cable at the RCA connector, and then connect the other or "hot" side of the twisted pair to the appropriate connection of the RCA connector. This gives you the shielding and RFI rejection of both the twisted pair and the ground shield. Think of this as a double ground if you like. A number of manufacturers make microphone cable, below are some examples.

    • Mogami 2534 Neglex
    • Canare Star Quad
    • Belden, my personal favorite is Belden #8422.

    These have a heavy copper-braid ground shield, which I do prefer to other types. Typical prices range from 65 to 75 cents per foot in 50-foot rolls. If you need help assembling these cables, feel free to contact me at banquer [at] erols.com.

    Shielded speaker cable? Definitely! Belden #8718 is a 12-AWG twisted pair with a foil shield and drain wire for an easy connection to earth ground. Connect the 12-AWG twisted pair to the appropriate amp and loudspeakers terminals. The drain wire should be connected to earth ground at one point only. I have it easy here in my listening room because my power amp chassis is connected to earth ground. Do not connect the drain wire to the speaker. If you don't have a readily available earth ground on your equipment, I suggest running the drain wire to the earth ground of your a.c. line. There are special a.c. socket plugs that have an earth ground connection only.

    If you are connecting the drain wire to earth ground, do not connect that drain wire to either wire of the twisted pair. Also, don't leave the drain wire unconnected as it can act as a very good receiving antenna.

    Shielding your speaker cables will greatly reduce static electricity that can form on cables when they rest on a carpeted floor, especially on cold, dry days. Shielded a.c. line cords can also help under strong interference conditions.

    Belden #17660 a.c. line cord has a foil and braid shield. This is an I.E.C. style power cord, 18 AWG and is typically priced from $12 to $16 per line cord.

    Ferrite Beads

    Ferrite BeadsSome people have recommended that ferrite beads or filters be used to reduce the influence of RFI or EMI particularly where small signal levels (such as from a phono cartridge) or wide-band inputs (such as with some preamps or amps) are concerned The editor of this rag even told me of a case of RF interference when he was trying out a particular maker's tube amp and preamp. The amp was turned on, the preamp off, and both were connected by a 30-foot run of ungrounded, unshielded, RCA-terminated interconnects. The signal from a radio station located about two miles from his house was clearly audible, though not loudly, but was reduced to a very low level with a pair of ferrite clip-on's placed on the interconnects near the amp. He tells me further that he knows of other cases where these clip-ons have been used with positive results on speaker cables. It seems obvious that both those interconnects and the speaker cables were acting like antennas. The editor also had a sort of "break-in" interference from a linear amp applied to a CB radio that busted into his FM listening due to a diode antenna effect with dirty nickel-plated RCA interconnects. Twisting them in the socket cured the problem, but a good gold-plated plug and jack system is obviously a better solution.

    Ferrite beads have also been recommended at the input of both amplifiers (particularly wideband ones) and preamp phono inputs, where the bead is strung around the signal wire just as it comes out of the RCA input jack or the clip-on type put around the interconnect. This bead can be put on by an advanced do-it-yourselfer who is handy with a soldering iron, but some current RCA jacks are soldered straight to the PC board so such an addition can't be made. The idea here is that the bead forms a filter that effectively blocks both RFI and EMI from entering the following circuitry. I am not so certain that this is a good idea in every case, as it seems to me that the filter might not block super high RF or electromagnetic interferences, which seem to be so much more common these days, or frequencies below 1 MHz such as AM radio.

    So you're probably asking yourself, did all of these fixes work? I haven't heard break-through from my ham radio neighbor in years. My telephone line is still susceptible; I'll have to work on that.

    Note: If you're having trouble locating Belden wire, try QPL Electronics Distributors at: (978) 671-9473, ask for Dave Dragon.

    This article is reprinted with the kind permission of Eugene Pitts III and The Audiophile Voice. Please visit The Audiophile Voice at www.audiophilevoice.com The author of this article would also like to thank Gene for his great editing, and being a real class act.


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